This invention relates to the formation of optimised sonar channels, more particularly in an all-round passive sonar system equipped with an advantageously circular listening base. The formation of the channels is obtained by the "charge-coupled device" (CCD) technique.
A preformed channel is used to listen for acoustic noise about a given direction. It is obtained by compensating the relative delays (in relation to this listening direction) of the electrical signals supplied by the transducers forming the listening base. These delays are obtained by delay lines and, in particular, by digital or analog CCD, shift registers.
The advantage of using charge-coupled devices rather than digital devices is attributable to the fact that, where charge-coupled devices are used, the signals are treated in the form of analog samples. As a result, hardware is simplified because, in digital systems, it is necessary to use as many shift registers as there are bits resulting from the quantization of the amplitudes. In addition, the CCD technique eliminates the need for analog-digital and digital-analog converters which are expensive and reduce the signal-to-noise ratio.
In this technique of forming channels by charge-coupled devices, the signals received by the various transducers forming the listening base may be treated in parallel. The disadvantage of this is that it requires as many CCD shift registers as there are transducers. Accordingly, it is of greater advantage to treat these signals in series by multiplexing.
For treating the signals in series according to the prior art, the delays are obtained by contacts on the CCD shift register which receives the multiplexed signals. These contacts supply sampled signals corresponding to different transducers with the delays required for forming the channels. In fact, for conventional bases having diameters of the order of 2 meters for example, these delays cannot be obtained with sufficient precision to enable the resulting channel reception patterns to be used in practice. This is due to the fact that, to obtain high precision, it would be necessary to use CCD shift registers having a number of cells too large (over 500) to be useable in view of the "transfer inefficiency" of charge-coupled devices.